A rotary vane pump is a positive-displacement pump that consists of vanes slidably mounted to a rotor that rotates inside of a cavity formed by a pump casing. In some cases, the vanes can be of variable length and/or spring-biased to maintain contact with the inner wall of the cavity as the rotor rotates. The simplest rotary vane pump includes a cylindrical rotor rotating inside of a larger cylindrical cavity. The axes of these two cylinders are offset, causing an eccentricity. Vanes are allowed to slide into and out of the rotor and seal against the inner wall of the cavity, creating rotating vane chambers disposed between two vanes. The rotor may engage or be disposed close to the inner wall at one point which creates a single pump chamber extending around the rotor and from a single inlet to a single outlet.
However, other rotary vane pumps are designed with an elliptical cavity formed in the pump casing with a cylindrical rotor. The rotor may engage or be disposed close to the inner wall at two points along opposite ends of the minor axis of the ellipse, which creates two pump chambers (or “twin pump chambers”) on opposite sides of the minor axis and along the portions of the inner wall near the major axis. Such a design includes two inlet/outlet pairs, one for each pump chamber.
At the inlet of each pump chamber, the inner wall that defines the cavity extends away from the rotor and causes the vanes to extend outward as the vane chambers increase in volume as the rotor and the vane chambers rotate away from the inlet towards the outlet. As the vane chambers pass the inlet, the vane chambers are filled with fluid drawn in through the inlet at inlet pressure, which may be atmospheric. At the outlet of the pump chamber, the inner wall extends towards the rotor, the vanes retract and the vane chambers therefore decrease in volume as the vane chambers rotate to the outlet, forcing the fluid out of the pump. For pumps with twin chambers, the above process is repeated twice for each rotation of the rotor. With a constant inlet pressure, the vane chambers deliver the same volume of fluid with each rotation. Multistage rotary vane vacuum pumps can attain pressures as low as 10−3 mbar (0.1 Pa).
The elliptical/twin chamber rotary vane pump design allows both sides of the rotor to generate pressure or vacuum, thus achieving greater flow in a smaller package size and because the pump chambers are disposed 180° from each other, side loading of the rotor is virtually eliminated. However because the cycle of each vane chamber of an elliptical vane pump is only 180° of rotation as opposed to 360° for a single chamber vane pump, at higher vacuum and pressure duties, there is not enough angular distance to effectively compress the fluid before it is exhausted without restricting the flow and increasing vane loading. The result is a pump having shorter vane life and that becomes louder, hotter, and less efficient as the pressure or vacuum is increased.
Similarly, like the twin chamber rotary vane pump design, the internal compression of single chamber rotary vane pumps is limited by the angular distance between the inlet and exhaust ports. Therefore, achieving higher pressure duties in single chamber rotary vane pumps also adversely affects sound levels, efficiency, heat, and vane life.